This invention relates to a mounting structure for mounting transducers on the surface of a fluid-containing conduit, and more particularly relates to a novel mounting structure which improves the signal-to-noise ratio of the transducer signal output and which simplifies the location of spaced transducers relative to one another.
Flowmeters which employ clamp-on transducers are well known, and are described, for example, in U.S. Pat. Nos. 4,425,803, 4,373,401, 3,987,674 and 3,869,915, each in the name of Joseph Baumoel. The disclosures of these and all other prior art materials mentioned herein are expressly incorporated by reference.
In flowmeters such as those described in the above patents, two spaced transducers are clamped to the outer surface of a conduit. One transducer transmits a train of pulses through the pipe or conduit wall, through the fluid and back through the pipe wall to the other transducer. The other transducer, in turn, transmits a signal through the fluid in the conduit and back to the first transducer. In the direct mode, the transducers are on opposite sides of the conduit. In the reflect mode, the transducers are on the same side, and the signal reflects off the facing inner wall of the conduit as it passes from one transducer to the other. In either mode, the difference in the time taken for sonic signals to travel first upstream and then downstream of the conduit relative to the direction of flow of fluid within the conduit is a measure of the flow velocity of the material within the conduit.
A clamping structure for holding transducers on the surface of the conduit or pipe is described in Baumoel, U.S. Pat. No. 4,425,803, referred to above. When mounting such transducers, they should be located a known distance apart along the axis of the conduit, which distance depends upon the conduit material, the conduit size, and the fluid within the conduit, among other parameters. As disclosed in above-mentioned U.S. Pat. No. 4,425,803, an index scale is provided on the transducer mounting track and the edges of the transducers are located at particular index numbers determined by the manufacturer and the transducers are then firmly clamped in place. This technique requires a skilled operator who can find the correct index marking for each transducer and requires that the transducer be held firmly in place while it is being clamped at the location in which it was set on the index scale.
As will be described, the present invention provides a novel positioning means for accurately positioning transducers in predetermined axial locations along the conduit length to a high degree of accuracy by untrained persons.
It is also well known that, during the operation of spaced transducers in the reflect mode, some of the pulse energy from the transmitting transducer will propagate axially along the pipe wall toward the receiving transducer. This direct pulse reaches the receiving transducer earlier than the pulse energy which traverses the fluid. Conventionally, the receiving transducer is gated so that it receives energy only during the period that the sonic energy through the fluid could be expected to be received. Therefore, the direct pulse through the pipe is not expected to interfere with the reflect-mode measurement taking place, with respect to the time of arrival of the sound traversing the fluid at the second transducer.
However, although the peak energy of the initial pulse which passes through the pipe has passed long before the reflect-mode signal which traverses the fluid arrives at the receiving transducer, there is nevertheless a "ringing" or oscillation through the pipe wall, so that some of the energy of the direct wall pulse is received by the receiving transducer at the time the energy traversing the fluid reaches the second transducer. This residual pipe noise, while small, adversely affects the signal-to-noise ratio of the measurement.
The structure of the present invention eliminates or considerably reduces the effect of energy which traverses between the transducers directly through the pipe wall when making a measurement in the reflect mode of the time taken for sonic energy to pass through the fluid within the conduit.